This invention relates to a semipermeable membrane that offers a high molecular transmission rate selectively to low molecular weight polar molecules while providing excellent resistance to pressure driven liquid flow. The composite membrane is generally useful as a barrier to liquid penetration while simultaneously allowing rapid molecular diffusion of certain polar molecules through the membrane. The high flux of low molecular weight polar molecules afforded by the semipermeable membrane is of value in separation applications where it is desired to concentrate a liquid by removal of low molecular weight polar molecules and/or where removal and/or collection of a low molecular weight species is desired. One particularly suitable use for this composite semipermeable membrane is in the area of rain protective garments.
Protective garments for wear in rain and other wet conditions should keep the wearer dry by preventing the leakage of liquid water into the garment and by allowing perspiration to evaporate as vapour from the wearer to the atmosphere. In the past, and through a long history of rainwear development, certain truly waterproof materials have not allowed the evaporation of perspiration, so that a physically active wearer became sweat-soaked. Materials such as oilskins, polyurethane coated fabrics and poly(vinyl chloride) films are waterproof but do not allow satisfactory evaporation of perspiration.
"Breathable" materials that do permit evaporation of perspiration have tended to allow liquid water penetration from the rain and thus are not truly waterproof. Fabrics treated with silicone, fluorocarbon, and other water repellants usually allow evaporation of perspiration but are only marginally waterproof; they allow water to leak through them under relatively low pressures, and usually leak spontaneously when rubbed or mechanically flexed. Rain garments must withstand the impingement pressure of falling and windblown rain, and the water pressures that are generated in folds and creases in the garment, as well as through other uses generating pressure, such as a pack strap and kneeling or sitting of the wearer.
It is widely recognized that garments should be "breathable" to be comfortable. However, it is not necessary that air pass through the garment for it to be comfortable, only that water from perspiration be transmitted from inside to outside so that undergarments do not become wet and so that the natural evaporative cooling effect can be achieved. The greater ability the material has to transmit water vapour the larger the comfort range of that garment. "Breathability" and the ability to transport interior moisture vapour to the external environment are used interchangeably in this discussion.
The transport of water through a layer can be achieved in numerous ways. Wicking is the most common when large quantities of moisture are to be transferred. Wicking materials ar those that promote the movement of liquid water via capillary action from the interior surface to the exterior surface where it evaporates. They are porous, with interconnecting pores that provide pathways through the wicking material.
Although some wicking materials may resist low pressure flow of liquid water through them due to the tortuousity and the overall length of the flow path, they also readily transport water by capillary action from the exterior surface to the interior surface and therefore prove unsuitable for rain material. The comfort attributed to cotton garments in warm climates results from the ability of such garments to transport water to the exterior surface to readily evaporate and provide cooling. Another natural wicking material, leather, owes its great comfort to breathability via wicking.
U.S. Pat. No. 3,953,566 assigned to the same assignee as the present application, provided porous membranes satisfying the two requirements of being liquid waterproof while also being permeable to the transport of water vapour. For rainwear, these membranes were usually laminated to various textiles for mechanical protection and style. The membranes are inherently hydrophobic and contain very small pores that resist the entry of liquid water even at substantial pressures or when rubbed or flexed, but they readily allow the flow of gases through them, including water vapour. Unlike wicking materials, breathability is achieved by evaporation of liquid water inside the garment or at the inner surface of the membrane followed by gaseous flow or diffusion of water vapour through the membrane's pores to the outside.
However, when such garments were worn during strenuous activities causing the wearer to perspire copiously, surface active agents in perspiration could gradually penetrate the hydrophobic membrane, coat its interior surfaces, thus lowering its apparent surface free energy which caused it to lose its waterproof characteristics and become a wicking material. To restore waterproofness, the garment required cleaning to remove the surface active contaminates. In practice, this was a significant drawback to widespread commercial acceptance of such garments.
Laminated materials which are both waterproof and breathable and which are especially suited for use in rainwear or tents are disclosed in U.S. Pat. No. 4,194,041, assigned to the same assignee as the present application, and specifically incorporated by reference herein. That invention provided a layered article, for use in waterproof garments or tents, that is waterproof, resistant to surface active agents in perspiration, and yet still permits the evaporation of perspiration and the transfer of moisture vapour through the layered article.
That invention comprises a combination of at least two layers: (1) an interior, continuous, hydrophilic layer that readily allows water to diffuse through it, yet prevents the transport of surface active agents and contaminating substances such as those found in perspiration; and (2) a hydrophobic outer layer that resists the flow of liquid water and simultaneously permits the transmission of water vapour therethrough, and also provides thermal insulating properties even when exposed to rain. Garments made of such materials are permanently waterproof from exterior water sources yet allow the evaporation of perspiration whenever the partial pressure of water vapour inside the garment exceeds that outside.
The hydrophilic layer used in the invention of U.S. Pat. No. 4,194,041 has a moisture vapour transmission rate by the test method set forth therein exceeding 1,000 gms/m.sup.2.multidot. 24 hours, and preferably above about 2,000 gms/m.sup.2.multidot. 24 hours. It permits no detectable transmission of surface active agents found in perspiration and preferably permits no detectable flow of liquid water at hydrostatic pressures up to 25 psig.
The hydrophobic layer used in that invention has a moisture vapour transmission rate, as measured by the test method set forth in that patent, exceeding 1,000 gms/m.sup.2.multidot. 24 hours and preferably exceeding 2,000 gms/m.sup.2.multidot. 24 hours; an advancing water contact angle exceeding 90.degree. degrees, and is preferably formed of a porous hydrophobic polymer.
However, those skilled in the art have continued to seek semipermeable membranes, for use in rainwear and for other various separations, having a maximized transmission rate of the desired permeant with a simultaneous effective efficiency of separation from undesired species. In some separation practices the transmission rate of the permeant and the efficiency of separation can be tailored not only by the membrane used but also by the condition of the separation. In the area of separation, as applied to the breathable rainwear application where the environment is so variable, it is desirable to have a maximized transmission rate of water over as broad a range of conditions as possible. In general, the material described in U.S. Pat. No. 4,194,041 has a water vapour transmission rate lower than that of the hydrophobic layer alone. It would be desirable to achieve a water vapour transmission rate comparable to the hydrophobic layer utilized alone while still achieving the desired protection of the hydrophobic layer provided by the hydrophilic layer used. The greater the capability of a garment material to transmit water vapour, the greater is the comfort range of that garment. Therefore, to those skilled in the art, there has been a continued effort to maximize the comfort range of a material by maximizing its transmission rate of water vapour throughout all conceivable conditions.
Likewise, those skilled in the art of membrane technologies have engaged in an ongoing, concerted effort to develop satisfactory membranes for commercial separation applications (reference, Membranes In Separations and as Supports, Volume 1, Report Text, Business Communications Company, Incorporated, 1983).
In the development of new membranes, the flux of the desired permeant, selective semipermeability, mechanical strength and service life are criteria to be considered. Much of the technology has focused on achieving sufficient flux at economically suitable conditions to make an application commercially viable. In all membrane separations it is necessary to have the semipermeability adjusted so that the flux is not only sufficiently high for the desired permeant, but also that separation efficiency is sufficient to achieve the desired end results. For commercial viability, a membrane having sufficient flux and selectivity must have sufficient strength to be able to be handled and used in actual applications. Also, numerous additional physical and chemical considerations must be considered to provide a membrane with a reasonable service life. It is with these four considerations in mind, as well as the required economic considerations, that those skilled in the art have continued an ongoing effort to develop suitable commercially viable semipermeable membranes and commercially viable processes to produce those membranes to be used in membrane separation technology.
To maximize the flux of the permeant it is desirable to keep the thickness of the membrane to a minimum without compromising continuity. The membrane should have the highest possible diffusivity of the desired permeant through the membrane.
If the desired permeant is water, it would be desirable to improve the solubility of water in the membrane by maximizing the hydrophilic nature of the chemical composition of the material and maximizing the amorphous content of the polymer. Historically, however, it has been difficult to obtain the required mechanical strength in a highly hydrophilic polymeric membrane to allow it to be useful commercially. Specifically, films of a highly hydrophilic polymer have tended to be weak and either easily torn or damaged by abrasion and/or flex or the stresses imposed upon them internally, especially when swollen with water. Accordingly, there is a need to increase the hydrophilicity of these membranes without concomitant deterioration in physical properties.
The requirement for chemical stability must be considered as it relates to service life in addition to the need for sufficient mechanical strength existing in a thin, highly hydrophilic membrane. For instance, with water as the permeant, it is recognized that water will be everywhere present and therefore the polymer must be resistant to hydrolysis to the degree required by the application. It is well-known that ethers offer good resistance to hydrolysis. As well as resistance to the various modes of degradation encountered in a membrane application, it is important that the polymer be non-soluble to the systems encountered, or else service life will be brief. If the polymer is inherently soluble in the system in consideration, for example a highly hydrophilic polymer exposed to an aqueous environment, insolubility may be introduced to the overall system by crosslinking the polymer into an infinite three-dimensional network. The same four considerations that were discussed above must be applied in considering a crosslinked polymer in its entirety. For instance, it is known that crosslinking a polymeric network generally decreases the flux of the desired permeant as crosslink density increases. Conversely, selectivity typically improves. Also, the functional groups involved in crosslinking must be chemically stable to degradation as required by the application (similar to the considerations of the base polymer).
The addition of thiols to olefins to form thioethers is a reaction that is well-known in the art. The thioether functionality affords chemical stability and hydrophilicity in many respects similar to those of its oxygen counterpart. This homolytic addition of the thiyl radical to carbon unsaturation proceeds by a free radical chain transfer mechanism and has been known to be photochemically induced. See, for example, Mechanism and Structure in Organic Chemistry, Gould, Edwin S., Henry Holt and Company Incorporated, 1959, pp 741ff, and Reactions of Organosulfur Compounds, Block, Eric, Academic Press, Incorporated, 1978, pp 207ff.
This chemical reaction has been reduced to commercial practice as described in U.S. Pat. Nos. 3,662,023, 3,661,744 and 3,708,413, and also as generally described in Journal of Polymer Sciences,Polymer Chemistry Edition, Volume 15, Morgan, Magnota and Ketley, 1977, pp 627 to 645. The teachings of the prior art disclose the utility of utilizing the Thiol/Ene chemistry for the curing of polymeric networks by a commercially viable processing technique, specifically as disclosed in U.S. Pat. No. 3,661,744, employing ultraviolet radiation.
U.S. Pat. No. 3,661,744 discloses a photocurable liquid polymer composition including a polyene containing at least two unsaturated carbon-to-carbon bonds at terminal positions having the structure ##STR1## a polythiol component containing two or more pendant or terminally positioned -SH groups, and a photocuring rate accelerator. The patent discloses that the combined functionality of the terminal or pendant -ene and thiol groups must exceed 4 to obtain a solid crosslinked product. The -ene and thiol components are disclosed to be free of reactive internal unsaturation. The patent discloses a process for forming a solid polythioether which comprises admixing the three components recited above as being present in the composition, and then exposing the mixture to actinic light.
U.S. Pat. No. 2,392,294 discloses a photochemical reaction under the influence of ultraviolet radiation to directly form thioethers from (a) hydrocarbons containing terminally unsaturated carbon atoms and (b) aliphatic mercaptans. The unsaturated hydrocarbons may be branched chain, or substituted derivatives thereof, which preferably contain two or more unsaturated linkages of aliphatic character. Included among the specific unsaturated hydrocarbons disclosed are polyolefins and olefin polymers, pentadiene-1,3, pentadiene-1,4, hexadiene-1,5, methylvinyl acetylene, divinyl ether, diallyl ether, dimethallyl ether, allyl alcohol and the like. The patent discloses that it is desirable to have terminal aliphatic unsaturated linkages in the unsaturated organic compound. The preferred polythiol component of this reference comprises dimercaptans of the general formula HS (CH.sub.2).sub.n SH. The polymethylene dimercaptans are disclosed to be capable of reacting with aliphatic hydrocarbons containing a plurality of unsaturated linkages to produce thioethers having a high molecular weight. Preferred unsaturated organic compounds which may be employed with these dimercaptans include the unsaturated compounds containing unsaturated linkages of aliphatic character in alpha and omega positions (terminal positions). The reference teaches the use of branched chain hydrocarbons or substituted derivatives thereof provided such compounds contain at least one, preferably two or more, unsaturated linkages of aliphatic character and that the most effective wavelengths of light for initiating the desired addition of mercaptans lie in that portion of the spectrum below about 3,200 Angstrom units and more particularly below 2,900 Angstrom units.
U.S. Pat. No. 3,454,539 discloses the reaction of certain polymercaptans with olefin-containing epoxides to produce highly functional polyepoxides. The reference discloses forming polythioepoxides by the reaction of polymercaptan with certain olefin-containing epoxides to improve the properties of the epoxide. This reference teaches that compounds containing more than two active olefin groups are not suitable for practicing the invention because of excessive activity of mercaptan groups with active olefin groups giving in many instances insolubility (crosslinking) without involving the epoxide groups in the reaction.
U.S. Pat. No. 3,466,336 discloses the reaction of acetylenic compounds and H.sub.2 S to form polythioethers. The three forms of polythioethers formed by the reaction disclosed include thiol-terminated polythioethers and vinyl-terminated polythioethers. Reactions of the polymers with trifunctional or polyfunctional molecules to form three-dimensional polymer networks are described. Polythioethers having vinyl sulfide end groups (SCH=CHR) can be reacted with tri and/or polythiols. This reference discloses the following reaction: ##STR2##
Free Radicals in Solution, Cheves Walling, John Wiley & Sons, Incorporated (New York) 1967 contains a Section describing three mechanisms for the addition of thiols to olefins, and principally describing a free radical chain reaction to form polymers. The book teaches that peroxides and light will initiate the reaction, and that acetone acts as a photosensitizer and permits light of over 3,000 Angstroms wavelength to act as a photoinitiator. The free radical chain reaction is taught to be useful with virtually any olefin not containing obviously inhibiting functional groups. Polymers from olefins and dithiols, formed, e.g., by the reaction ##STR3## are disclosed to have been studied, with molecular weights of 60,000 being achieved.
U.S. Pat. No. 3,708,413 discloses high energy curable compositions that are curable to provide a solid, self-supporting cured product in the presence of high energy bombardment.
U.S. Pat. No. 3,592,798 discloses the preparation of liquid polythiol products having three or more pendant or terminal groups per molecule that combine with polyfunctional organic reagents such as diacrylates, dimaleates, etc. to form stable, three-dimensional networks. Also disclosed is the basic polyene/polythiol addition reaction which is used to produce a liquid polythioether product. The use of gamma radiation to promote the polyene/polythiol addition reaction is disclosed.
U.S. Pat. No. 3,898,349 discloses a paint vehicle prepared from a composition comprising (1) about 98 to 2 percent by weight of a liquid polyene containing at least two terminal reactive unsaturated carbon-to-carbon bonds per molecule and (2) about 2 to 98 percent by weight of a polythiol containing at least two thiol groups per molecule, the total combined functionality of (a) the reactive unsaturated carbon-to-carbon bonds per molecule in the polyene and (b) the thiol groups per molecule in the polythiol being greater than 4, which composition is curable, preferably under ambient conditions, in the presence of a free radical generator such as electromagnetic radiation of wavelength of about 2,000 to 7,000 Angstroms or high energy ionizing radiation. In instances where the free radical generator is electromagnetic radiation, a curing rate accelerator in an amount ranging from 0.0005 to 50 percent by weight of said composition is added to the polyene/polythiol composition. The resultant vehicle, in pigmented form, can be used per se as a solventless paint or can be used with an inert organic solvent or as a water-soluble or water-dispersible paint.
Commercially available polythiols and polyenes and/or their packages do not meet the needs discussed above for the chemical composition of the desired final hydrophilic film of this invention. Specifically, they are not of a sufficient hydrophilic nature nor are they hydrolytically stable. Commercially available thiols have the thiol functional groups introduced into the desired polymer backbone through known hydrolytically sensitive functional groups such as esters, urethanes and ureas. Likewise, many of the commercial Thiol/Ene systems are used in conjunction with acrylate systems which also have the inherently hydrolytically unstable ester functional group.